What is chorionic villus sampling?
Chorionic villus sampling (CVS) is a prenatal test that diagnoses chromosomal abnormalities such as Down syndrome, as well as a host of other genetic disorders. The doctor takes cells from tiny fingerlike projections on your placenta called the chorionic villi and sends them to a lab for genetic analysis.
CVS and another test called amniocentesis produce a karyotype – a picture of your baby’s chromosomes – so that your caregiver can see for sure if there are any problems.
Women who choose to have CVS or amniocentesis are often those at increased risk for genetic and chromosomal problems, in part because these tests are invasive and carry a small risk of miscarriage.
The main advantage of CVS over amniocentesis is that you can have it done earlier — generally between 10 and 13 weeks of pregnancy. For an amnio, you’ll have to wait until you’re at least 16 weeks pregnant.
What kind of problems does CVS diagnose?
Like amniocentesis, CVS can identify:
- Nearly all chromosomal abnormalities, including Down syndrome, trisomy 13, trisomy 18, and sex chromosome abnormalities (such as Turner syndrome). The test can diagnose these conditions, but it can’t measure their severity.
- Several hundred genetic disorders, such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease. The test is not used to look for all of them, but if your baby is at increased risk for one or more of these disorders, CVS can usually tell you whether he has the disease.
Unlike amniocentesis, CVS cannot detect neural tube defects, such as spina bifida. If you opt for CVS, you’ll be offered a blood screening test in your second trimester to determine whether you’re at increased risk for neural tube defects. Most neural tube defects can be detected by a detailed second-trimester ultrasound done at a state-of-the-art academic center.
Be aware that if you have CVS, there’s a 1 to 2 percent chance of getting an unclear result. This is called a confined placental mosaicism, in which some of the cell lines cultured from the placenta contain abnormal chromosomes and some are normal. If your CVS detects a mosaicism, you’ll have to have amniocentesis and possibly other testing to determine whether your baby is affected.
Is there any way to reduce the risks of CVS?
Ask your practitioner or genetic counselor to refer you to an experienced doctor who does a lot of CVS procedures and who is expert at both the transabdominal and transcervical procedures, so your doctor can choose the procedure that’s safest for you. You may also want to ask about the estimated rate of procedure-related miscarriage for the doctor or the center where you’re considering having the procedure done.
You’ll also want to make sure that an experienced registered diagnostic medical sonographer provides continuous ultrasound guidance during the procedure. This greatly increases the chances that the doctor will be able to obtain enough tissue on the first try, so you won’t have to repeat the procedure.